In order to achieve precision cutting either to a precise depth into the workpiece, or completely through the workpiece and possibly to a precise depth into the work surface, it is desirable, in the case of flat sheet cutting, to produce a perfectly planar work surface and to guide the blade for movement at a constant height relative to the work surface. Such perfection cannot, of course, be achieved, and the closer to perfection a machine is made, the more expensive it is and the more care must be taken with it.
The invention is more particularly concerned with a machine of the type defined above which also includes means to adjust the height of the blade as the blade moves along the cutting path.
In one known machine of this type, an ultrasonic sensor mounted with the blade senses the height of the sensor above the upper surface of the workpiece and the height of the blade (and sensor) is adjusted on the basis of the output from the sensor. A disadvantage of this arrangement is that, if there are any small air pockets between the workpiece and the work surface, or if there are any small rucks in the workpiece, then these will be sensed and the height will be adjusted, but the workpiece will then be pressed flat against the work surface by the blade and an error will occur.
In another known machine of this type, an eddy current sensor mounted with the blade senses the height of the sensor above the work surface and the height is adjusted on the basis of the sensor output. However, a disadvantage of this arrangement is that the sensor output varies with variations in the bulk of the work surface support structure in the vicinity of the sensor, such as strengthening members of the support structure, and furthermore the sensor output is affected if the workpiece is of a conductive material such as metal or carbon fibre.
The present invention seeks to provide an apparatus of the type defined above in which the blade can cut to a precise height without requiring the work surface and blade movement to be perfectly planar and without suffering the disadvantages of the height adjustment systems described above.